A high-power device generating a great deal of heat due to high power consumption has the problem that the function of the device is deteriorated or stops, or the life span of the device is shortened, by an increased temperature of the device itself. Therefore, an apparatus for easily releasing a great deal of heat in the air, e.g., a heat sink, is mounted on the high-power device, which lowers the temperature by forced convection using a fan.
However, noise and power consumption caused by rotation of a fan should be taken into account in the method using the fan. That is, it is required to adjust the rotational speed of the fan depending on a real-time temperature of the high-power device so as to minimize the noise and the power consumption, rather than equally driving it at the same rotational speed regardless of the temperature of the high-power device.
Moreover, in a system having a high-power device, if the temperature exceeds a pre-determined limit, the operation of the entire system needs to be stopped in order to protect the system.
For this, it is necessary to know a temperature generated from the high-power device in advance in real-time, which is an essential requirement of a portable laptop computer. Therefore, a temperature measuring device for measuring the real-time temperature of a high-power device is mounted at a place adjacent to the high-power device. An example of such a measuring device includes a pad-type temperature measuring device attached on a Central Processing Unit (CPU) of a computer, wherein the temperature of the high-power device is measured in real-time, thereby adjusting the cooling capacity by a cooling apparatus such as a fan.
As described above, in case a temperature has to be monitored in real-time, and for a test for verifying whether the thermal design of the entire system is appropriate or not, the temperature of the high-power device needs to be measured. Thus, it should be checked whether a measured temperature exceeds a maximum temperature set to ensure the normal operation and life span of the high-power device. A conventional temperature measuring method for measuring the temperature of a high-power device as above will be explained below in detail.
FIG. 1 is a cross-sectional view schematically showing a temperature measuring device and a heat sink of a packaged high-power device. It shows a conventional method for measuring the temperature of a high-power device.
Referring to FIG. 1, a high-power device 11, which is an essential component of a semiconductor chip 10, is attached to the top surface of a die 13 by a thermally conductive adhesive 12. The high-power device 11 is electrically connected to an external lead 15 through a gold wire 14. Such a high-power device 11 is packaged by a mold compound 16.
A pad-type temperature measuring device 40 for measuring the temperature of the high-power device 11 is attached to the mold compound 16, and a thermally conductive paste 30 is applied to the top thereof, and then a heat sink 20 is placed thereon. The thermally conductive paste 30 is used to make the mold compound 16 and the heat sink 20 thermally well-contacted with each other. At this time, the temperature of the high-power device 10 is higher than the temperature measured by the pad-type temperature measuring device 40, and the difference therebetween is calculated by deriving a thermal resistance from the high-power device 10 to the pad-type temperature measuring device 40, as follows:Td=Tp+Rth,d-pP  Eq. (1)
wherein ‘Td’ is the temperature of the high-power device 11, ‘Tp’ is the temperature of the packaging measured by the pad-type temperature measuring device 40, and P is the power consumption of the high-power device 11. And, ‘Rth,d-p’ the thermal resistance from the high-power device 10 to the pad-type temperature measuring device 40, which is determined by an experiment or analysis.
In this case, however, the accuracy of the calculated temperature of the high-power device is low because of the inaccuracy of the thermal resistance, and the real-time temperature cannot be measured due to the time taken until the thermal energy generated from the high-power device 11 is transmitted to the pad-type temperature measuring device 40.
FIG. 2 is a perspective view showing a semiconductor device having an unpackaged high-power device mounted thereon. In FIG. 2, the reference numerals as in FIG. 1 designate the same or corresponding components.
Referring to FIG. 2, the high-power device 11 is generally attached to an alumina substrate 60, and the alumina substrate 60 is mounted to an aluminium housing 50. Since a temperature measuring device cannot be added to the unpackaged high-power device 11, a wire-type thermocouple 70 is attached to the alumina substrate 60 or to the aluminium housing 50 adjacent thereto. In this case, however, there also exist the inaccuracy of the temperature of the high-power device 11 and the delay of heat transfer, as in FIG. 1. The degree of errors and delay in heat transfer become more serious because there are several steps from the high-power device 11 to a measuring position, including the high-power device 11, an adhesive (not shown) for adhering the high-power device 11 to the alumina substrate 60, the alumina substrate 60, an adhesive (not shown) for adhering the alumina substrate 60 to the aluminum housing 50, the aluminum housing 50, and the thermocouple 70.
Meanwhile, the portion of the high-power device from which most parts of heat are generated is a transistor. Therefore, the performance and life span of the high-power device differs depending on the temperature of the transistor. By the way, regarding the temperature distribution in the high-power device, the size of the high-power device is very small, but the temperature distribution therein is not uniform. That is, while the temperature of a transistor that generates much heat is very high, the temperature of other regions is slightly different from the temperature of the ambient environment, which shows the phenomenon of a local temperature increase. Especially, in case of a high-frequency device using gallium arsenide (GaAs) as a dielectric.
Thus, even if the temperature of the bottom surface of the device has been measured by the method explained in FIGS. 1 and 2, the temperature of the transistor desired to be found ultimately is calculated as:Tj=Tbase+Rth,j-bP  Eq. (2)
wherein ‘Tj’ is the temperature of the transistor, ‘Tbase’ is the temperature of the bottom surface of the device, and ‘p’ is the power consumption of the device. Also, ‘Rth,j-b’ the thermal resistance from the transistor to the bottom surface of the device, which is determined by an experiment or analysis and provided by the manufacturer of the device.
A representative method, among the methods for obtaining the thermal resistance of an unpackaged high-power device by an experiment, is a method of photographing the temperature distribution of a high-power device by an infrared microscope. That is, this method is to photograph the temperature distribution of the surface of a high-power device while normally operating. However, this method is problematic in that, if the transistor is screened by an air bridge, the temperature of that portion cannot be found, and as the resolution of a microscope does not reach the minimum line width of the device, it is not possible to measure an accurate temperature. Like this, because even a measured thermal resistance may differ according to individual devices, a thermal resistance greater than the actual value is generally provided in consideration of the safety factor. As a result, the temperature calculated with thermal resistance cannot be the accurate temperature of the transistor.
The above description is summarized as follows. In the conventional method for measuring the temperature of a high-power device, if it is desired to know the maximum temperature of a high-power device regardless of whether it is a packaged high-power device or unpackaged high-power device, it is actually difficult to directly measure the maximum temperature from the high-power device in reality. Thus, the maximum temperature is calculated by measuring the temperature of the packaging of the device or its ambient temperature and then obtaining, by an experiment or analysis, the thermal resistance from the portion showing the maximum temperature to the portion where the temperature is actually measured.
However, the conventional method for measuring the temperature of a high-power device has some problems that it is difficult to obtain an accurate maximum temperature because of a difficulty in obtaining an accurate thermal resistance, and a real-time temperature cannot be found since there exists a delay in heat transfer between a maximum temperature point and a temperature measurement point. Additionally, there is a problem that the step of an experiment or analysis for obtaining a thermal resistance is added.